Polyester is a polymeric material made from the esterification of polybasic organic acids with polyhydric alcohols. One exemplary polyester is manufactured by reacting dimethyl terephthalic acid with ethylene glycol so as to result in a compound chemically known as polyethylene terephthalate and commonly identified as PET. Polyesters are currently used as a base material in a wide variety of applications. For example, polyester is commonly used to make photographic films, x-ray films, bases for magnetic coating such as in recording tapes, beverage containers, surgical aids such as synthetic arteries, and as a fabric for making garments and other similar items. However, although polyester is very useful, the waste materials containing polyester are beginning to create a waste management and disposal problem.
Currently, those skilled in the art are seeking different methods for recovering and reusing polyester contained in waste PET plastic products. However, recovery of polyester from waste products has been difficult. In particular, many of the prior art processes are not capable of efficiently or economically recovering polyester due to non-PET contaminants and low densification.
Fundamentally, the process of converting polyethylene terephthalate to a polyester polyol involves the process for "cracking back". This "crack back" process is essentially one of heating the polyethylene terephthalate in the presence of excess amounts of glycols, alcohols, or other suitable polyols, such that a retrodegradation of the polymer molecule chain takes place. In the extreme case, monomer units of the acids and glycol can be produced. Lower molecular weight oligomers consisting of one to approximately twenty glycol and acid units depending on the desired properties of the end product is the goal of this conversion process.
This chemical process is generally referred to as transesterification whereby an excess of glycol is added and chemically attacks the ester bonds in the PET so as to break the existing ester bonds. Immediately thereafter, form new ester bonds form between the attacking glycols and the acid moieties which remain from cleaving the first ester bonds. The glycols of choice for such a process are diethylene glycol and propylene glycol. However, any glycol, alcohol or polyol with liquid temperatures less than the softening/melting temperature of the PET (approximately 175.degree. to 250.degree. C.) can be employed. The net result of this chemical reaction as the process proceeds is to change the ratio of reacted terephthalic acid to reacted glycol in the PET polymer from approximately 1.0 in the original starting material toward a theoretical limit of 0.5. In general, the final ratio will be in the range of 0.5 to 0.75.
This chemical process is shown chemically hereinbelow: ##STR1## The rate of reaction is a function of the temperature, pressure and the degree to which the excess glycol and PET can be brought into intimate contact with one another. The process will proceed optimally at and beyond the temperature point where the PET has been converted to a melting stage in the presence of a suitable glycol raised to that same temperature. This is in the range of 175.degree. to 250.degree. C. Higher temperatures will increase the rate, but at a minimum, a melted state for the PET is needed in order to get commercially acceptable rates of reaction. For lower boiling glycols and alcohols, such as propylene glycol, the reaction vessel must be capable of maintaining pressure in order to permit heating these glycols to the minimum acceptable range of 230.degree. to 250.degree. C.
Given that acceptable temperatures and pressures can be achieved, the limiting parameter for the process becomes the degree to which intimate contact between the reactants can be maintained. For the PET crack back process, this is especially important. The free fall density of the PET raw material itself will generally be significantly less than that of the glycols used as the other reactant. The form of the PET can vary from very thin films and fiber types to granulated, flaked or pelletized types. The film and fiber types are the lowest density forms and the flaked, pelletized and granulated types are of the highest density forms. The use ratios between the PET and the glycols are typically in the range of about 60% to 150% by weight of glycol to the weight of the PET. In examples where the amount of glycol and PET by weight are equal, the volume occupied by the glycol may be from 20% to 60% of the volume of the PET depending on the free fall density of the PET which depends on its form. In examples of product where the amount of glycol needs to be less by weight than the weight of PET, the relative amount of volume occupied in a reaction vessel naturally becomes less.
In the prior art, so as to carry out such crack back processing, a vertical reactor has been employed with an agitator employing several sets of impeller blades so as to mix the ingredients and to keep the contents homogeneously dispersed. The typical vertical processing reactor has heat exchange from an outside source to the reactor contents by use of a hot oil or steam jacket mounted on the sidewalls and possibly the bottom cone of the reactor or by use of an internal coiled tube to convey the hot oil or steam mounted along the sidewalls of the reactor. In either case, the heat source for the reactor contents is mainly, if not exclusively, along the sidewalls of the reactor. Thus, the ability to provide good efficient heat transfer to the contents depends to a very significant degree on the agitator mixing and dispersing action such that materials heated at the sidewalls are continuously replaced by unheated materials away from the sidewall. This dispersing of the heat along with the contents is necessary so as to create the greatest efficiency in the heating process in addition to keeping the mixture homogeneous.
The major difficulty with crack back processing of PET by these conventional prior art vertical reactors is that the low density PET is both difficult to disperse by typical agitation and that the PET itself acts as a very poor conductor of heat. Thus, under typical conditions, when the necessary charges of PET and glycol are added together, the glycol portion occupies only a small portion of the space at the bottom of the reactor and the solid PET fills from the bottom of the reactor to the uppermost level. The glycol/PET lower occupied portion of the reactor, being a liquid with mixed solids, can be agitated provided that the lower impellers reach that lower level of the reactor. The "PET only" occupied portion of the reactor cannot be effectively agitated and, therefore, the heat produced from the sidewalls cannot be readily dispersed to those portions of the contents away from the sidewalls. Furthermore, the relatively low density PET material is a very poor conductor of heat. The net effect of these difficulties is that the PET contents in the upper areas of the reactor are very slow to heat and reach the melt point needed to make the process effective. The mixed glycol/PET contents in the lower part of the reactor can be effectively heated and possibly agitated. This becomes the only region of the reactor that is effectively participating in the chemical process of cracking back the PET. As this region completes the crack back process, and the excess glycol and now liquid cracked back polyol in that region forms, the PET from the upper regions of the reactor can become incorporated into this now-liquid region and can be cracked back as well. This becomes a very protracted process and is not very favorable to commercial conversions.
In the past, various patents have issued relating to the process for converting PET into polyesters. U.S. Pat. No. 4,602,046 discloses a method for the recovery of polyester from scrap material, such as photographic film, having a polyester base. The scrap material is cut or chopped into small individual pieces or flakes and treated in a caustic alkaline solution at a solids level of at least 25% by volume and under conditions of high shear.
U.S. Pat. No. 3,652,466 discloses another process of recovering the polyester from polyester films. The coated films are cut into small pieces and treated with a caustic aqueous alkali solution to form a slurry. The slurry is fed into a classification column in which the pieces move downward countercurrent to a moving column of aqueous liquid which separates the pieces from the coating material. The pieces are removed from the bottom of the column in suspension and can thereafter be used as a source of polyester material.
U.S. Pat. No. 5,395,858, issued on Mar. 7, 1995 to J. A. Schwartz, Jr. describes a process for recycling polyester contained in waste materials. The polyester is converted into ethylene glycol and terephthalic acid. The process includes first combining materials containing polyester with an alkaline solution so as to form a slurry. The slurry is heated so as to cause ethylene glycol to evaporate. The remaining product stream is then mixed with water and filtered to remove any undissolved impurities. The aqueous filtrate can be acidified causing terephthalic acid to precipitate. U.S. Pat. No. 5,580,905, issued to the same inventor, describes a similar process.
It is an object of the present invention to provide an apparatus and method which effectively and efficiently converts waste PET materials into an polyester polyols.
It is another object of the present invention to provide an apparatus and method which facilitates the ability to intimately mix the glycol with the PET on the interior of the reactor vessel.
It is another object of the present invention to provide an apparatus and method which assures the optimum recovery of polyester polyols from a charge of PET and glycol.
It is still another object of the present invention to provide a method and apparatus which facilitates the loading of scrap PET material into the reactor vessel.
It is still another object of the present invention to provide a method and apparatus which more effectively transmits heat thoroughly throughout the mixture of glycol and PET on the interior of the reactor vessel.
It is still another object of the present invention to provide a method an apparatus which is easy to use, easy to manufacture and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.